The present invention relates to an improved cyclic, endothermic hydrocarbon conversion process and to a catalyst bed system for accomplishing the same. Specifically, the improved process comprises contacting a hydrocarbon feedstock with a multi-component catalyst bed, wherein the catalyst bed comprises a first component that is a catalyst specifically designed to convert the hydrocarbon feed to a predetermined product or product mix, and a second component that generates heat after being exposed to reducing and/or to oxidizing reaction conditions.
Several endothermic hydrocarbon conversion processes are utilized in commercial operations. These processes include the Houdry cyclic fixed bed dehydrogenation process, the fluid bed paraffin dehydrogenation process, the fluid bed ethylbenzene dehydrogenation process, and fluid bed catalytic cracking process, among others. Because these processes are endothermic, heat must be consumed from the surroundings in order for the hydrocarbon conversion reaction to occur. In each of these processes, at least one reaction is promoted by contacting a hydrocarbon feed with a catalyst. Further, in each of these processes there is at least one reducing and/or oxidizing reaction that regenerates the catalyst. The heat needed for the endothermic reactions to occur is provided in part by combustion of coke and other undesirable side products that deposit on the catalyst during the conversion process. This combustion takes place during the regeneration process. However, additional heat is normally needed and this is provided by hot air or steam that is fed into the catalyst bed from external sources between the hydrocarbon conversion cycles.
As an example, in the typical Houdry dehydrogenation process as taught in U.S. Pat. No. 2,419,997, an aliphatic hydrocarbon passes through a dehydrogenation catalyst bed. As the aliphatic hydrocarbon passes through the catalyst bed, the hydrocarbon is dehydrogenated to its complementary olefin. The olefin is then flushed from the catalyst bed, the catalyst is regenerated and reduced, and the cycle is repeated. This dehydrogenation reaction is highly endothermic. Therefore, during the dehydrogenation step, the temperature near the inlet of the catalyst bed (where the aliphatic hydrocarbon initially enters the catalyst bed) can decrease by as much as 100° C. This decrease in temperature causes a decrease in hydrocarbon conversion. In addition, during the dehydrogenation step, it is common for coke to form and deposit on the catalyst, further reducing the activity of the catalyst.
In order to reheat the catalyst bed and to remove the coke that has deposited on the catalyst, the reactor is purged of hydrocarbon and then undergoes a regeneration step with air heated to temperatures of up to 700° C. Heat is provided to the bed by the hot air that passes through the bed and also by the combustion of the coke deposits on the catalyst. Reduction of the catalyst, with a reducing gas such as hydrogen, prior to the dehydrogenation step also provides some heat. During regeneration, the hot air flows from the inlet of the catalyst bed to the outlet. This regeneration cycle is normally relatively short, so there is a tendency for the inlet of the bed to be significantly hotter than the outlet of the bed, but because of the timing between cycles in the Houdry dehydrogenation process, the catalyst bed does not have time to equilibrate thermally. Thus, the outlet section of the bed remains cooler than the inlet section of the bed as aliphatic hydrocarbon is again fed into the reactor. The high temperature at the inlet of the bed tends to cause the formation of undesirable by-products and thus lowers selectivity and yield of the desired olefin. On the other hand, the lower temperature at the outlet of the bed does not allow full utilization of the catalyst and thus the olefin yield is lower than would be otherwise expected or desired. Also, because the coke distribution in the catalyst bed is not an independently controlled parameter, the heat distribution is also not easily controllable within the bed. Each of these factors affects the resulting catalyst bed temperature profile and makes control of the temperature profile in the bed difficult.
In U.S. Pat. No. 2,423,835, Houdry teaches that the catalyst bed temperature may be controlled within a temperature range suitable for the reactions without requiring an extraneous heating or cooling fluid to be circulated through or around the reaction chamber by including within the catalyst bed “inert” material capable of absorbing or storing up heat which can subsequently be released as desired or required. In commercial practice for fixed bed reactors, this is typically achieved by using a physical mixture of a dehydrogenation catalyst and a granular, alpha-alumina “inert” material as the catalyst bed. Although the addition of the inert material provides a reversible heat sink for the process, and helps stabilize the overall temperature swings in the reactor, the inert is not capable of providing extra heat for the process nor can it produce heat during any stage of the process. Hence, an external heat source is still required even with the combined use of the catalyst and the inert.
The challenge is to identify a commercially feasible means for controlling the temperature profile within the catalyst bed of an endothermic process. Ideally, any such means will allow for heat addition to predetermined sections of the catalyst bed without using a catalytically active material that produces large quantities of unwanted side products.